1. Field of the Invention
This invention relates to a sensor to detect faults in a ferrous object and, more particularly, to a sensor to detect magnetic flux leakage from a discontinuity in a magnetized object. The invention will be described in relation to the detection of a crack in a ferrous pipe, although the invention is broader in scope.
2. Description of the Prior Art
In the prior art there are numerous systems for detecting faults or cracks in ferrous objects and more particularly ferrous pipes.
One accepted method of inspection for faults is by an ultrasonic fault detector. There are many brands and types of ultrasonic fault detectors on the market. The ultrasonic transducer requires a couplant, such as, water or glycerine, to conduct the ultrasound wave into the metal and return the echo. Thus, a full length scan of a pipe around 360 degrees of circumference by the ultrasonic method is both slow and. messy. The echo signal that is returned by ultrasonic examination of a cylindrical object, a pipe, is made complex by the multiple reflections from the inside and outside walls. Considerable experience is required to interpret the signal data to confirm the existence of a fault. Background ultrasonic noise, generated by plant equipment, masks the signal generated by the fault.
Another method of fault detection is monitoring magnetic flux leakage. A fault in a ferromagnetic material may be detected if that material has been magnetized and the fault causes a discontinuity in the path through which the magnetic field travels. The fault causes a perturbation in the magnetic field around the test piece. Two devices are used to detect perturbations in a magnetic field, a coil of wire and a Hall Effect semiconductor. Both detectors provide rapid detection of a fault. The output voltage which the coil of wire produces is dependent on the strength of the magnetic field and the velocity with which the coil passes through the field. The output of the Hall Effect semiconductor device depends only on the strength of the field. The Hall Effect sensor produces an output voltage proportional to the magnetic field in which it rests even if the sensor is mechanically static. However, the Hall Effect semiconductor may not function well if the magnetic field is not strong enough.
Another system of fault detectors uses magnetic field pattern detectors or sensors arranged near the surface of the ferrous bodies but the outputs of the sensors are connected in a specific sequence and in a time-division multiplex manner to an evaluation circuit. Such a fault detector is disclosed in U.S. Pat. No. 4,763,070 but the circuit is of such complexity as not to be of commercial value. An earlier version of this type detection system may be found in U.S. Pat. No. 4,538,108.
The use of a Hall cell to detect faults is described in U.S. Pat. No. 3,816,766. This patent concerns mainly the problem of temperature sensitivity of the Hall Circuit. In this type of Hall detector, the magnetic field may not be strong enough to cause the detector alone to be useful under production conditions requiring rapid and accurate responses.